Small digital cameras integrated into mobile (cell) phones, personal digital assistants and music players are becoming ubiquitous. Each year, mobile phone manufacturers add more imaging features to their handsets, causing these mobile imaging devices to converge towards feature sets and image quality that customers expect from stand-alone digital still cameras. Concurrently, the size of these handsets is shrinking, making it necessary to reduce the total size of the camera accordingly while adding more imaging features. Optical Zoom is a primary feature of many digital still cameras but one that mobile phone cameras usually lack, mainly due to camera height constraints in mobile imaging devices, cost and mechanical reliability.
Mechanical zoom solutions are common in digital still cameras but are typically too thick for most camera phones. Furthermore, the F/# (“F number) in such systems typically increases with the zoom factor (ZF) resulting in poor light sensitivity and higher noise (especially in low-light scenarios). In mobile cameras, this also results in resolution compromise, due to the small pixel size of their image sensors and the diffraction limit optics associated with the F/#.
One way of implementing zoom in mobile cameras is by over-sampling the image and cropping and interpolating it in accordance with the desired ZF. While this method is mechanically reliable, it results in thick optics and in an expensive image sensor due to the large number of pixels associated therewith. As an example, if one is interested in implementing a 12 Megapixel camera with X3 ZF, one needs a sensor of 108 Megapixels.
Another way of implementing zoom, as well as increasing the output resolution, is by using a dual-aperture imaging (“DAI”) system. In its basic form, a DAI system includes two optical apertures which may be formed by one or two optical modules, and one or two image sensors (e.g., CMOS or CCD) that grab the optical image or images and convert the data into the electronic domain, where the image can be processed and stored.
The design of a thin MAI system with improved resolution requires a careful choice of parameters coupled with advanced signal processing algorithms to support the output of a high quality image. Known MAI systems, in particular ones with short optical paths, often trade-off functionalities and properties, for example zoom and color resolution, or image resolution and quality for camera module height. Therefore, there is a need for, and it would be advantageous to have thin MAI systems that produce an image with high resolution (and specifically high color resolution) together with zoom functionality.
Moreover, known signal processing algorithms used together with existing MAI systems often further degrade the output image quality by introducing artifacts when combining information from different apertures. A primary source of these artifacts is the image registration process, which has to find correspondences between the different images that are often captured by different sensors with different color filter arrays (CFAs). There is therefore a need for, and it would be advantageous to have an image registration algorithm that is more robust to the type of CFA used by the cameras and which can produce better correspondence between images captured by a multi-aperture system.